The goal of plant breeding is to combine in a single variety or hybrid various desirable traits. Major objectives in sunflower breeding include improved seed yield, earlier maturity, shorter plant height, uniformity of plant type, and disease and insect resistance. High oil percentage is important in breeding oilseed types whereas large seed size, a high kernel-to-hull ratio, and uniformity in seed size, shape, and color are important objectives in breeding and selection of nonoilseed sunflower. Other characteristics such as improved oil quality, protein percentage and protein quality are also important breeding objectives.
Field crops are bred through techniques that take advantage of the plant's method of pollination. A plant is self-pollinated if pollen from one flower is transferred to the same or another flower of the same plant. A plant is cross-pollinated if the pollen comes from a flower on a different plant.
Plants that have been self-pollinated and selected for type for many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny. A cross between two different homozygous lines produces a uniform population of hybrid plants that may be heterozygous for many gene loci. A cross of two plants each heterozygous at a number of gene loci will produce a population of hybrid plants that differ genetically and will not be uniform.
Sunflower (Helianthus annuus L.), can be bred by both self-pollination and cross-pollination techniques. The sunflower head (inflorescence) usually is composed of about 1,000 to 2,000 individual disk flowers joined to a common base (receptacle). The flowers around the circumference are ligulate ray flowers with neither stamens nor pistil. The remaining flowers are hermaphroditic and protandrous disk flowers.
Natural pollination of sunflower occurs when flowering starts with the appearance of a tube partly exerted from the sympetalous corolla. The tube is formed by the five syngenesious anthers, and pollen is released on the inner surface of the tube. The style lengthens rapidly and forces the stigma through the tube. The two lobes of the stigma open outward and are receptive to pollen but out of reach of their own pollen initially. Although this largely prevents self-pollination of individual flowers, flowers are exposed to pollen from other flowers on the same head by insects, wind, and gravity.
A reliable method of controlling male fertility in plants offers the opportunity for improved plant breeding. This is especially true for development of sunflower hybrids, which relies upon some sort of male sterility system. Two types of male sterility, genetic and cytoplasmic, have been found in sunflower.
Hybrid sunflower seed is typically produced by a male sterility system incorporating genetic or cytoplasmic male-sterile (CMS) inbreds. Plants of a CMS inbred are male sterile as a result of factors resulting from the cytoplasmic, as opposed to the nuclear, genome. Thus, this characteristic is inherited exclusively through the female parent in sunflower plants, since only the female provides cytoplasm to the fertilized seed. CMS plants are fertilized with pollen from another inbred that is not male-sterile. Pollen from the second inbred may or may not contribute genes that make the hybrid plants male-fertile.
Plant breeding methods involving genetic or cytoplasmic male sterility, or induction of male sterility by gibberellic acid, allow for complete hybridization of lines and hence greater precision in estimating combining ability. Various tester parents and tester schemes are being used. A. V. Anaschenko has conducted extensive testing for general combining ability by the top cross method with chemical emasculation of the female parent with gibberellic acid. He has used open pollinated cultivars, hybrids, and inbred lines as testers. A. V. Anaschenko, The Initial Material for Sunflower Heterosis Breeding, Proceedings of the 6th International Sunflower Conference, 391-393 (1974). B. Vranceanu used a monogenic male sterile line as a female parent to test for general combining ability and subsequent diallel cross analysis with artificial emasculation to test for specific combining ability. B. Vranceanu, Advances in Sunflower Breeding in Romania, Poc. 4th International Sunflower Conference (Memphis, Tenn.), 136-146 (1970). Recent testing by breeders in the United States has included the rapid conversion of lines to cytoplasmic male sterility by using greenhouses and winter nurseries and subsequent hybrid seed production in isolated crossing blocks using open pollinated cultivars, synthetics, composites, or inbred lines as tester.
There are several methods of conferring genetic male sterility available, such as multiple mutant genes at separate locations within the genome that confer male sterility. According to A. I. Gundaev, Prospects of Selection in Sunflower for Heterosis, Sb. Rab. Maslichn. Kult., 3:15-21 (1966), genetic male sterility first was reported in the Soviet Union by Kuptsov in 1934. Since then, numerous investigators have reported genetic male sterility in sunflower. Vranceanu indicated isolation of more than thirty sources of male sterility in the Romanian program, most of which were controlled by a single recessive gene. Diallel cross analysis of ten of these lines indicated the presence of five different genes. The studies of E. D. Putt and C. B. Heiser were some of the first reported to assess the value of genetic male sterility to produce hybrid seed. They concluded that lines of partial male sterility may have the most immediate value in commercial production of hybrid seed as not only could the partial male sterile lines hybridize well in crossing plots, they could also be increased and easily maintained. E. D. Putt and C. B. Heiser, Jr., Male Sterility and Partial Male Sterility in Sunflowers, Crop Science, 6:165-168 (1966).
In order to produce hybrid seed using complete genetic male sterility, the male sterile locus must be maintained in the heterozygous condition in the female parent. This is accomplished by sib pollinations of male sterile plants (ms ms) with heterozygous male fertile plants (Ms ms) within the female parent. The resultant progeny from the male sterile plants will segregate 1:1 for fertile and sterile plants. When such lines are used in hybrid seed production the fertile plants must be removed prior to flowering to obtain 100% hybridization with the male parent line.
Production of hybrid seed by the genetic male sterile system has the advantage that fertile hybrid plants can be produced using any normal male fertile line as the male parent. Although removal of the male fertile plants was facilitated greatly by the discovery of a close linkage between genes for genetic male sterility and anthocyanin pigment in the seedling leaves, the high labor cost required to remove the male fertile, anthocyanin pigmented plants from the female rows of seed production field is a disadvantage of the genetic male sterile system. In addition, the requirement to incorporate and maintain the link characters in the female parent is another disadvantage of the genetic male sterile system. P. Leclercq, Une sterilite male utilisable pour la production d hybrides simples de tournesol, Ann. Amelior. Plant 35 16:135-144 (1966).
The genetic male sterility system has been replaced largely by the cytoplasmic male sterile and fertility restorer system in most current hybrid sunflower breeding programs. The value of genetic male sterility now appears to be primarily an alternate method of hybrid seed production should problems develop with the use of cytoplasmic male sterility such as occurred in maize with susceptibility to southern corn leaf blight. The system also may have value for developing suitable testers for evaluating inbred lines, and subsequent production of hybrid seed for testing.
Around 1960, the first reports of cytoplasmic sterility indicated that most crosses of cytoplasmic male sterile plants with normal male fertile lines produced progeny with variable percentages of sterile plants. Varying degrees of partial sterility were also reported. Through selection and test crossing, lines that produced 92-96% sterile progeny were developed and utilized in experimental production of hybrid seed. A. I. Gundaev, Prospects of selection in sunflower for heterosis, Sb. Rab. Maslichn. Kult., 3:15-21 (1966) and I. A. Gundaev, Basic principles of sunflower selection, Genetic Principles of Plant Selection, p. 417-465 (1971). Leclercq in France reported the discovery of cytoplasmic male sterility from an interspecific cross involving H. petiolaris Nutt. and H. annuus L. This source of cytoplasmic male sterility was shown to be very stable. For more information regarding sunflower breeding and genetics, see Gerhardt N. Fick, and Jerry Miller, The Genetics and Breeding of Sunflower, Sunflower Science and Technology, pages 441-558 (1997) incorporated herein by reference.
Cytoplasmic male sterile lines are traditionally developed by the backcrossing method in which desirable lines that have undergone inbreeding and selection for several generations are crossed initially to a plant with cytoplasmic male sterility. Thereafter the inbred line to be converted is used as a recurrent parent in the backcrossing procedure. The final progeny will be genetically similar to the recurrent parent except that it will be male sterile.
Fertility restorer lines are developed by transferring a dominant restorer gene to an established inbred line with normal cytoplasm by backcrossing. If this procedure is used, selected plants must be crossed to a cytoplasmic male sterile line after each generation to determine if the fertility restorer genes are present. A more common procedure is self-pollination and selection of male fertile plants from commercial hybrids or planned crosses of parents having restorer genes in male sterile cytoplasm. This procedure does not require test crossing to a male sterile line during selection because the plants will be fully male fertile if the necessary restoring genes are present.
Typically most fertility-restorer lines in use today have restorer genes in male sterile cytoplasm, are resistant to downy mildew and have recessive branching. The later trait extends the period of pollen production and is useful in obtaining simultaneous flowering with female lines in hybrid seed production fields. Restorer lines RHA271, RHA273, and RHA274 were the first such lines to be developed and have been used widely in producing hybrids in breeding programs throughout the world.
Other methods for conferring male sterility are also available and could be used in developing male sterile and fertility restoring sunflowers. For example Albertsen et al., of Pioneer Hi-Bred, U.S. patent application Ser. No. 07/848,433, have developed a system of nuclear male sterility in corn which could also be used in sunflower which includes: identifying a gene which is critical to male fertility; silencing this native gene which is critical to male fertility; removing the native promoter from the essential male fertility gene and replacing it with an inducible promoter; inserting this genetically engineered gene back into the plant; and thus creating a plant that is male sterile because the inducible promoter is not "on" resulting in the male fertility gene not being transcribed. Fertility is restored by inducing, or turning "on", the promoter, which in turn allows the gene that confers male fertility to be transcribed.
There are many other methods of conferring male sterility in the art of plant breeding and any method can be used, each with its own benefits and drawbacks. These methods use a variety of approaches such as delivering into the plant a gene encoding a cytotoxic substance associated with a male tissue specific promoter or an antisense system in which a gene critical to fertility is identified and an antisense to that gene is inserted in the plant (see: Fabinjanski, et al. EPO 89/3010153.8 publication no. 329,308 and PCT application PCT/CA90/000037 published as WO 90/08828)
Development of Sunflower Inbred Lines
The use of male sterile inbreds is but one factor in the production of sunflower hybrids. The development of sunflower hybrids requires, in general, the development of homozygous inbred lines, the crossing of these lines, and the evaluation of the crosses. Pedigree breeding and recurrent selection breeding methods are used to develop inbred lines from breeding populations. Breeding programs combine the genetic backgrounds from two or more inbred lines or various other broad-based sources into breeding pools from which new inbred lines are developed by selfing and selection of desired phenotypes. The new inbreds are crossed with other inbred lines and the hybrids from these crosses are evaluated to determine which of those have commercial potential.
There are many important factors to be considered in the art of plant breeding, such as the ability to recognize important morphological and physiological characteristics, the ability to design evaluation techniques for genotypic and phenotypic traits of interest, and the ability to search out and exploit the genes for the desired traits in new or improved combinations. Such methods have also evolved to assist in breeding programs. The use of DNA markers such as restriction fragment length polymorphisms and randomly amplified polymorphic DNA's (RAPDS) are powerful tools of genetic analysis and have been used extensively in a number of species. Linkages of molecular markers with important agronomic traits such as cyst nematode resistance in potato, powdery mildew resistance in barley, insect resistance in long bean have been established. Markers are also correlated with other plant characteristics like flower color, plant height and fertile period response. Sunflower molecular marker technologies are in the early stages of development and isozyme polymorphisms have been used to characterize inbred lines and will be a valuable tool in assisting breeders with selection.
The objective of commercial sunflower hybrid line development programs is to develop new inbred lines to produce hybrids that combine to produce high yields and superior agronomic performance. The primary trait breeders seek is yield. However, many other major agronomic traits are of importance in hybrid combination and have an impact on yield or otherwise provide superior performance in hybrid combinations. Major objectives in sunflower breeding include improved seed yield, improved seed oil percentage and oil quality, earlier maturity, shorter plant height, uniformity of plant type, and disease and insect resistance. In addition, the lines per se must have acceptable performance for parental traits such as seed yields and pollen production, all of which affect ability to provide parental lines in sufficient quantity and quality for hybridization. These traits have been shown to be under genetic control and many if not all of the traits are affected by multiple genes.
The trait of primary economic importance in sunflower yield exhibits considerable genetic variability and is often associated with other traits, such as stem fasciation, trichome length, serration of leaf martin, and chlorotic leaf color to name a few. Inbred lines which are used as parents for breeding crosses differ in the number and combination of these genes. These factors make the plant breeder's task more difficult. Compounding this is evidence that no one line contains the favorable allele at all loci, and that different alleles have different economic values depending on the genetic background and field environment in which the hybrid is grown. Fifty years of breeding experience suggests that there are many genes affecting yield and each of these has a relatively small effect on this trait. The effects are small compared to breeders' ability to measure yield differences in evaluation trials. Therefore, the parents of the breeding cross must differ at several of these loci so that the genetic differences in the progeny will be large enough that breeders can develop a line that increases the economic worth of its hybrids over that of hybrids made with either parent.
Pedigree breeding starts with the crossing of two genotypes, each of which may have one or more desirable characteristics that is lacking in the other or which complements the other. If the two original parents do not provide all the desired characteristics, other sources can be included in the breeding population. In the pedigree method, superior plants are selfed and selected in successive generations. In the succeeding generations the heterozygous condition gives way to homogeneous lines as a result of self-pollination and selection. Typically in the pedigree method of breeding five or more generations of selfing and selection is practiced: F.sub.1.fwdarw.F.sub.2 ; F.sub.3.fwdarw.F.sub.4 ; F.sub.4.fwdarw.F.sub.5, etc.
A single cross hybrid sunflower variety is the cross of two inbred lines, each of which has a genotype that complements the genotype of the other. The hybrid progeny of the first generation is designated F.sub.1. In the development of hybrids only the F.sub.1 hybrid plants are sought. Preferred F.sub.1 hybrids are more vigorous than their inbred parents. This hybrid vigor, or heterosis, can be manifested in many polygenic traits, including increased vegetative growth and increased yield.
The development of a hybrid sunflower variety involves three steps: (1) the selection of plants from various germplasm pools for initial breeding crosses; (2) the selfing of the selected plants from the breeding crosses for several generations to produce a series of inbred lines, which, although different from each other, breed true and are highly uniform; and (3) crossing the selected inbred lines with different inbred lines to produce the hybrid progeny (F.sub.1). During the inbreeding process in sunflower, the vigor of the lines decreases. Vigor is restored when two different inbred lines are crossed to produce the hybrid progeny (F.sub.1). An important consequence of the homozygosity and homogeneity of the inbred lines is that the hybrid between a defined pair of inbreds will always be the same. Once the inbreds that give a superior hybrid have been identified, the hybrid seed can be reproduced indefinitely as long as the homogeneity of the inbred parents is maintained.
A single cross hybrid is produced when two inbred lines are crossed to produce the F.sub.1 progeny. A double cross hybrid is produced from four inbred lines crossed in pairs (A.times.B and C.times.D) and then the two F.sub.1 hybrids are crossed again (A.times.B).times.(C.times.D). A three-way hybrid is produced from three inbred lines. Two inbreds are crossed (A.times.B) to create an F1 hybrid, which is then crossed to a third inbred (A.times.B).times.C. Much of the hybrid vigor exhibited by F.sub.1 hybrids is lost in the next generation (F.sub.2). Consequently, seed from hybrid varieties is not used for planting stock.
It has been shown that most traits of economic value in sunflower are under the genetic control of multiple genetic loci, and that there are a large number of unique combinations of these genes present in elite sunflower germplasm. If not, genetic progress using elite inbred lines would no longer be possible. Much progress has been made in the improvement of sunflower. Over the last 50 years Russian breeders were able to increase seed oil content from about 300 grams/kg in the 1930's to over 500 grams/kg among current cultivars. The introduction of adapted and tested hybrids in the USA in the 1970's was estimated to have resulted in yield increases in excess of 25% as well as significant improvements in disease resistance in agronomic type. Extensive to genetic variation is available in sunflowers and breeders are optimistic that new lines with superior combining ability, agronomic type, seed quality traits, and/or disease and insect resistance can be developed. Also the wild species of Helianthus offer tremendous resources of genetic diversity for further improvement.
Biotechnology has also received a great deal of attention as a basic technique for improving sunflower. Embryo culture techniques have been developed which have greatly facilitated crossing with wild species. Regeneration of plants from tissue culture is being used to create new sources of genetic variability. Pugalici et al. 1991. "Plant regeneration and genetic variability from tissue cultures of sunflowers", Plant Breeding, 106:114-121. Foreign genes from several species including bean, maize, and Brazil nut have been transferred into sunflower using Agrobacterium tumefaciens. The production of double haploids by anther or microspore culture and the genetic mapping of valuable traits or genes using RFLP's are traditional biotechnology procedures that are useful to breeders.
Sunflower is an important and valuable field crop. Thus, a continuing goal of plant breeders is to develop high-yielding sunflower hybrids that are agronomically sound based on stable inbred lines. The reasons for this goal are obvious: to maximize the amount of seed produced with the inputs used and minimize susceptibility of the crop to environmental stresses. To accomplish this goal, the sunflower breeder must select and develop superior inbred parental lines for producing hybrids. This requires identification and selection of genetically unique individuals that occur in a segregating population. The segregating population is the result of a combination of crossover events plus the independent assortment of specific combinations of alleles at many gene loci that results in specific genotypes. Based on the number of segregating genes, the frequency of occurrence of an individual with a specific genotype is less than 1 in 10,000. Thus, even if the entire genotype of the parents has been characterized and the desired genotype is known, only a few if any individuals having the desired genotype may be found in a large F.sub.2 or S.sub.0 population. Typically, however, the genotype of neither the parents nor the desired genotype is known in any detail.
In addition to the preceding problem, it is not known how the genotype will react with the environment. This genotype by environment interaction is an important, yet unpredictable, factor in plant breeding. A breeder of ordinary skill in the art cannot predict the genotype, how that genotype will interact with various environments or the resulting phenotypes of the developing lines, except perhaps in a very broad and general fashion. A breeder of ordinary skill in the art would also be unable to recreate the same line twice from the very same original parents as the breeder is unable to direct how the genomes combine or how they will interact with the environmental conditions. This unpredictability results in the expenditure of large amounts of research resources in the development of a superior new sunflower inbred line.